如何根据列表中的 boolean 值从列表中删除元素，而不是按索引删除它们？

Question

I have a list of elements that contains booleans and strings. I have the list output matching what I want, however I need to make it able to delete based on the leading boolean value instead of index. This is the code I have now

public class collectionsPractice {

    public static void main(String[] args) {
        ArrayList fruits = new ArrayList();
        fruits.add(false);
        fruits.add("Strawberry");
        fruits.add("Pineapple");
        fruits.add(true);
        fruits.add("Watermelon");
        fruits.add("Lemon");
        fruits.add(false);
        fruits.add("Cantaloupe");
        fruits.add("Grapes");
        fruits.add("Coconut");
        
        fruits.remove(4);
        fruits.remove(4);
        
        System.out.println(fruits);
    }

}

The list before the removal looks like this:

[false, Strawberry, Pineapple, true, Watermelon, Lemon, false, Cantaloupe, Grapes, Coconut]

After the remove calls it looks like this:

[false, Strawberry, Pineapple, true, false, Cantaloupe, Grapes, Coconut]

I want the strings after every true boolean to be deleted. (I know the remove statements look weird, but I have to call them twice because after the first call the index of "Lemon" shifts down by one).

So instead of having the remove at some index, I want to loop through the list and see if the boolean value in the list is true or false. Then if false I want to keep going through the list. However, if it is true, I want to delete the strings in between that boolean and the next boolean. The output should look the same as the output I have with the two remove statements, I just want to be able to do that without having to know how many objects or items are in the list. Is there a way to do this? If any clarification is needed, please let me know in the comments.

Answer 1

This looks like a job for an Iterator :

boolean delete = false;

Iterator<?> iterator = (Iterator<?>) fruits.iterator();
while (iterator.hasNext()) {
    Object element = iterator.next();
    if (element instanceof Boolean b) {
        delete = b;
    } else if (delete) {
        iterator.remove();
    }
}

Answer 2

You may traverse list from the end to avoid mutation of the "tail", something like:

public static void main(String[] args) {
    ArrayList fruits = new ArrayList();
    fruits.add(false);
    fruits.add("Strawberry");
    fruits.add("Pineapple");
    fruits.add(true);
    fruits.add("Watermelon");
    fruits.add("Lemon");
    fruits.add(true);
    fruits.add(false);
    fruits.add("Cantaloupe");
    fruits.add("Grapes");
    fruits.add(true);
    fruits.add("Coconut");

    int end = fruits.size();
    for (int i = end - 1; i >= 0; i--) {
        if (Boolean.TRUE.equals(fruits.get(i))) {
            fruits.subList(i + 1, end).clear();
            end = i;
        } else if (Boolean.FALSE.equals(fruits.get(i))) {
            end = i;
        }
    }
    System.out.println(fruits);
}

Answer 3

Here's a possible implementation:

ArrayList fruits = new ArrayList();
fruits.add(false);
fruits.add("Strawberry");
fruits.add("Pineapple");
fruits.add(true);
fruits.add("Watermelon");
fruits.add("Lemon");
fruits.add(false);
fruits.add("Cantaloupe");
fruits.add("Grapes");
fruits.add("Coconut");
fruits.add("Lemon");

System.out.println(fruits); //before

ArrayList toRemove = new ArrayList();
boolean deletionFlag = false;

for(int i = 0; i < fruits.size(); i++){
    Object obj = fruits.get(i);
    if (obj instanceof Boolean){
        deletionFlag = (boolean)obj;
    }else{
        if(deletionFlag) toRemove.add(obj);
    }
}

fruits.removeAll(toRemove);
System.out.println(fruits); //after